Carbon and N turnover in moist sandy soil following short exposure to a range of high soil temperature regimes

Laboratory experiments were undertaken to examine the effects of high soil temperatures on N biotransformations in sandy soils. Soils were incubated at 30°, 40°, 50°, and 60°C for 2 days, before all treatments were kept at 30°C for up to 41 days. Another laboratory experiment evaluated the effect of different cycles of exposure to 50° and 30°C, including frequency and duration of exposure to 50°C, to assess the sensitivity of N biotransformations to temporary increases in temperature in the high range. CO₂-C production, soil microbial biomass-C, gross N mineralisation, gross N immobilisation, and potential gross nitrification were measured. Gross N mineralisation and CO₂-C production increased with temperature (in the range 30°-50°C) and exhibited a Q₁₀-relationship close to 2. Between 50° and 60°C, Q₁₀ was closer to 2.8. The increase in gross N mineralisation and CO₂-C production after exposure to 50° and 60°C is attributed to the decomposition of dead microbial biomass by the viable microbial population but this flush in activity was short-lived. Immobilisation rate was always low and remained unaffected by the temperature regime, probably because the growth of the microbial biomass was inhibited at the higher temperatures. This imbalance between gross N mineralisation and immobilisation resulted in rapid increases in mineral N in soil. Two 6-h cycles of 50°C interspersed with 30°C were equally as effective as a single 48-h exposure at stimulating CO₂ production. Evidence of uncoupling CO₂ production and gross N mineralisation was observed in one study where soil was incubated at 50°C, but this response was not universal. The nitrification process was totally suppressed by exposure to temperature higher than 40°C, probably due to thermal denaturation of enzymes. The relevance of findings to field conditions is discussed.